Warning systems that warn the driver upon departure from the lane or threatened departure from the lane are known in the prior art. For example, European Patent Application No. EP 1074430 indicates a procedure in which, on the basis of an image sensor system, the lane on which a vehicle is moving is determined and the driver is warned if the vehicle is leaving this lane or threatens to leave this lane. The precise boundary conditions under which such a warning takes place are not indicated. In particular, the respective driving situation is not taken into account, so that false warnings can occur, which is unpleasant for the driver.
From German Patent Application No. DE 196 37 053, it is known to use an angle-resolving distance and speed sensor, preferably a radar or laser sensor, to recognize objects on an adjacent lane, and to determine the direction of travel of these objects on the basis of their relative speed.